: SMVT is a tranporter which, when expressed in mammalian cells, induces the transport of pantothenate, biotin, and lipoate. Since SMVT mediates the uptake of biotin both in the intestine and the placenta, the investigator hypothesizes that the expression of SMVT in the placenta is also subject to regulation by plasma concentrations of biotin, pantothenate and lipoate. This will be tested in vitro and in vivo using BeWo choriocarcinoma cells and the pregnant rat as model systems. Studies will include analysis of changes in the transport activity, transporter density, and the steady-state levels of the transporter mRNA induced by changes in the concentrations of pantothenate, biotin, and lipoate in the culture medium or maternal plasma. Immunolocalization studies will be performed to examine the expression of the cloned transporter in the brush border and/or basal membrane of the syncytiotrophoblast to understand the role of SMVT in the entry and/or exit of the vitamins in the placenta. The ontogenic development of the transporter in placenta and fetus will be studied using rats as an animal model. A detailed functional characterization of two vitamin C transporters, SVCTI and SVCT2, is proposed to determine features of SVCT2 function that render the specific expression of this transporter in the placenta more advantageous for the transplacental transfer of vitamin C. Proposed studies will also investigate the regulation of SVCT2 expression in rat placenta and BeWo cells in response to substrate availability and ontogenic development of SVCT2 in rat placenta.